PART II. CHAPTER XXIV. 281 



Tests of Relative Age of Plutonic Rocks. 



tern. These trappean masses, says Mr. Murchison, must have 

 been evolved at intervals from volcanic fissures at the bottom of 

 the sea, when the sand, pebbles, and mud, now forming the ac- 

 companying sedimentary rocks, were deposited.* 



Professor Sedgwick, in his account of the geology of Cum- 

 berland, has described various trap-rocks which accompany the 

 green slates of the Cambrian system, beneath a limestone con- 

 taining organic remains. Different felspathic and porphyritic 

 rocks and greenstones occur, not only in dikes, but in conform- 

 able beds ; and there is occasionally a passage from these igne- 

 ous rocks to some of the green quartzose slates. Professor 

 Sedgwick supposes these porphyries to have originated contem- 

 poraneously with the stratified chloritic slates, the materials of 

 the slates having been supplied, in part at least, by submarine 

 eruptions oftentimes repeated. f 



CHAPTER XXIV. 



ON THE DIFFERENT AGES OF THE PLUTONIC ROCKS. 



Difficulty in ascertaining the precise age of a plutonic rock Test of age by 

 relative position Test by intrusion and alteration Test by mineral composition 

 Test by included fragments Recent and Pliocene plutonic rocks, why invisi- 

 ble Tertiary plutonic rocks in the Andes Granite altering Cretaceous rocks 

 Granite altering Lias in the Alps and in Sky Granite of Dartmoor altering Car- 

 boniferous strata Granite of the Old Red sandstone period Syenite altering 

 Silurian strata in Norway Blending of the same with gneiss Most ancient 

 plutonic rocks Granite protruded in a solid form On the probable age of the 

 granite of Arran, in Scotland. 



WHEN we adopt the igneous theory of granite, as explained 

 in the 9th chapter, and believe that different plutonic rocks have 

 originated at successive periods beneath the surface of the planet, 

 we must be prepared to encounter greater difficulty in ascertain- 

 ing the precise age of such rocks, than in the case of volcanic 

 and fossiliferous formations. We must bear in mind, that the 

 evidence of the age of each contemporaneous volcanic rock was 

 derived, either from lavas poured out upon the ancient surface, 



* Murchison, Silurian System, &c. p. 404. 

 t Geol. Trans. 2d series, vol. iv. p. 55. 



